I think 3 turns on the fact that this is more specific than just “information that would inhibit prosecuting the case.” That would include exculpatory evidence, which the prosecutor would have to turn over.
It's a soft requirement, because the witness will be 'necessary'[1] in court in the particular circumstances of this particular crime. If the prosecutor had a stronger case (Say, a different witness, or CCTV evidence, etc, etc, etc), then this particular witness would not be necessary.
[1] In the opinion of the average criminal lawyer, assuming that the case is litigated correctly, and that the jury isn't entirely composed of utter mouth-drooling morons prejudiced against the defendant.